Living tissue has relatively low absorbance in the near-infrared (700-1000 nm) range of the electromagnetic spectrum. Whenever photon absorbance is low, the likelihood that tissue will autofluoresce is also reduced markedly, and autofluorescence of living tissue in the NIR is minimal. Hence from an absorbance and autofluorescence standpoint, NIR light has distinct advantages. Scatter is significant at all wavelengths, although again, the absolute value of scatter tends to be lower in many tissues in the NIR.
In the last few years, there has been a dramatic rise in the use of imaging systems that exploit NIR light, and in some cases, exogenous NIR fluorophores. The three basic types of imaging systems are steady-state (e.g., continuous wave (CW)), time-domain, and frequency-domain systems. Within each system, source/detector geometry can be based on point or area illumination, and point or area detection, leading to over twelve different imaging system configurations.
Paralleling the explosion in imaging systems is a similar rise in the use of exogenous NIR fluorophores. A comprehensive review of NIR fluorophores used for in vivo imaging has been published recently (Frangioni et al., 2003). Briefly, NIR fluorescent contrast agents now exist for vascular mapping, perfusion mapping, sentinel lymph node mapping, quantitating protease activity, imaging cell injury, imaging tissue response to injury, and imaging tumors.
To develop and to test imaging systems, such as near-infrared fluorescence imaging systems, for both research and clinical use, tissue-like “phantoms” are necessary. Phantoms are non-living models of living tissue that attempt to recapitulate or mimic the optical behavior of the tissue.
Although many tissue-like phantoms have been described (see, e.g., Cubeddu et al., 1997; Wagnieres et al., 1997; Quan et al., 1993; Kelly et al., 1998; Giller et al., 2003; Boehm et al., 2001; Jiang et al., 2003), they typically have one or more of the following problems: 1) they can be difficult to construct; 2) they often have an index of refraction mismatch between a fluorescent target and the tissue-like medium; 3) they do not permit simple insertion of fluorescent objects at any depth in the phantom; 4) they do not permit a simple adjustment of background absorbance and scatter independently and with physiological relevance; 5) they cannot be constructed, tested, and then shipped to another imaging system for direct comparison; 6) they have a low stability of both fluorescent target and tissue-like material over time; 7) they lack precision in their ability to quantitate fluorescence yield of target; 8) their geometries are not suitable to both ring and planar imaging system geometries; and 9) they do not provide the ability to create isotropic and non-isotropic volumes in the same phantom.
As imaging systems and contrast agents converge, there is an increasing need for standardized NIR fluorescent phantoms that can be compared among laboratories, and that assist with the training of surgeons, and other users, on new imaging systems.